Effects of oxygen-fuel ratio on combustion stability of a model rocket engine with hypergolic propellant

To estimate the effects of oxygen-fuel (O/F) ratio on the combustion stability of a model rocket engine with hypergolic propellant, experiments were carried out at different O/F ratios in a rectangular model engine with dual-liquid swirl coaxial injectors. The pressure oscillations and CH*-characterized heat release pulsations in the combustion chamber were simultaneously recorded by high-frequency pressure sensors and photomultiplier tubes (PMT). The effects of O/F ratio on combustion stability were obtained. The results show that the low-frequency oscillation of 41 Hz occurs in fuel mainfold after combustion initiation, which induces synchronous low-frequency oscillations in the combustion chamber during shutdown. In the process of increasing O/F ratio from 0.933 to 1.789, the combustion chamber undergoes a combustion stability transition process of stability, mild instability, first-order transverse instability, and second-order transverse instability. The amplitude of pressure oscillations in the 2W mode is only 4.69% of the mean combustion chamber pressure. By incorporating the PMT signal, it is found that the coupling of pressure and heat release signals is more obvious when combustion instability is more intense. Rayleigh index analysis based on the experimental data shows that the driving source of combustion instability in the 1W mode is mainly located at both sides of the combustion chamber, while suppression is found in the middle of the combustion chamber. The analysis suggests that the generation of combustion instability may be related to the interaction between the propellant and the combustion chamber walls.